Drought, beetles and fire in Colorado

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Since 2004, southwestern Colorado's Englemann spruce forests have experienced a severe outbreak of spruce beetles (Dendroctonus rufipennis). Such outbreaks occur episodically, multiple times per century (Anderson et al. 2010). Early research in northern Colorado linked a prior outbreak that occurred in the 1940s and 1950s to wind damaged trees that favored growth in the population of the spruce beetle. More recent research suggests that while local factors may still be important, outbreaks are linked to multi-year drought, which is in turn correlated with sea surface temperatures of the North Atlantic (Hart et al. 2013). On the attached slides, these relationships are conveyed for drought in south central Colorado where a vast number of trees have succumbed to spruce beetles during the current extended drought and outbreak.

Aerial photos and Forest Health surveys indicate that infestation and mortality is a multi-year process that gradually thins stands over several years. This slow fine-scale patchy mortality differs from what occurred in 2013 during the West Fork Complex (http://www.inciweb.org/incident/3436/). These wildfires burned a portion of stands that had previously experienced spruce beetle kill with mixed effects.

The interactive effects of multiple disturbances in this landscape are complex (Bebi et al. 2003), however the combined effects of physiological stress from drought, beetle kill and wildfire appear to be transforming this forest into something far different than what it was a decade ago.